An operator of medical services, who operates an X-ray CT apparatus to image an object such as a patient, sometimes punctures a needle in paracentesis and attaches his or her hand in order to prevent the object from moving during a scan. An operator takes appropriate action to avoid his or her exposure so that direct rays from an X-ray tube are not directed to him or herself, and also has to pay attention to indirect exposure of scattered rays caused by scattered X-rays which were irradiated on an object.
In an X-ray CT apparatus, a half scan is known as a technique to acquire data of one volume of CT image to be reconstructed. In a half scan, X-rays are irradiated while rotating the X-ray emission source around an object by 180 degrees plus a fan angle. Exposure of a half scan is smaller than exposure of a scan over whole of a rotational orbit. However, exposure in a half scan changes according to the relationship between the standing point of an operator and the irradiation section of X-rays on the rotational orbit.
Some of conventional X-ray CT apparatuses display a scanned image of an object and an image of an estimated irradiation range of X-rays by superimposing one of them on the other, in order to check whether the exposure range of the object includes a needless region or not. However, in such a conventional X-ray CT apparatus, it is impossible to recognize which span on the rotational orbit X-rays are irradiated during a half scan. Thus, in such a conventional X-ray CT apparatus, it is impossible to determine the standing position where exposure to an operator is minimized and the position where an operator attaches his or her hand.